This invention relates to a device for detecting nitrogenous, phosphoric, chloric and/or oxygenous substances inside an object, particularly explosives or addictive substances in pieces of luggage. According to the invention, a nuclear activating device stimulates the emission of positron radiation from nitrogen, phosphorous, chlorine and/or oxygen, and an activity measuring device records positron-electron annihilation radiation.
A device of this generic type is disclosed in European Patent Document EP 0 358 237 A1, in which pieces of luggage to be examined are guided on a conveyor belt through a radiation chamber and there are exposed to X-ray radiation (gamma radiation) of between 10.6 and 13.0 MeV. If nitrogenous substances are contained in the piece of luggage, indicating the presence of explosives, the X-ray radiation converts the .sup.14 N-atoms, into radioactive .sup.13 N-atoms, which emit positron radiation. When emitted positrons impinge on electrons, a known annihilation radiation of 0.511 MeV results. After being subjected to this type of radiation, the pieces of luggage are then transported to scintillation counters to detect the presence of any such released annihilation radiation. On the basis of the signals of the scintillation counters, conclusions are then drawn by means of a computer concerning the nitrogen concentration in the piece of luggage. However, the known arrangement furnishes no information on the spatial distribution of the nitrogenous substances in the piece of luggage, and can therefore supply only very rough information on the presence of explosives.
A more exact spatial resolution concerning the distribution of nitrogenous substances is possible by means of the device disclosed in European Patent Document EP 0 218 240 A2. However, this device is based on the activating process .sup.14 N (gamma, 2n) .sup.12 N, for which X-ray radiation of at least 30.64 MeV is required. For this reason, the X-ray unit generates a bremsstrahlung of 35 to 40 MeV which, in a narrow beam, is used to scan the piece of luggage passing through on the conveyor belt. The resulting annihilation radiation is then recorded by means of two opposite detector lines on the basis of a coincidence measurement, and conclusions can be drawn from the geometrical relationships between the exciting X-ray beam and the detectors recording two isochronous 0.511 MeV photons concerning the location of a nitrogenous substance and the concentration of the nitrogenous substance present at that location in the piece of luggage.
It is an object of the present invention to provide a device suitable for detecting nitrogenous substances inside a piece of luggage as well as phosphoric, chloric and oxygenous substances, which permits a more exact spatial resolution of these substances than previously possible, and requires an activation energy that is as low as possible.
This object is achieved by the detection device constructed according to the invention, which is based on the generation of .sup.13 N-atoms from nitrogenous substances by subjecting them to X-ray radiation (bremsstrahlung) or gamma radiation with energies of from 10 MeV up to a maximum of 14 MeV. By means of a known coincidence measuring device, a first rough activity density distribution can then be derived for the examined piece of luggage. The piece of luggage is X-rayed by means of a separate X-ray unit which may be connected in front of or behind the activity measuring device, and a three-dimensional absorption density distribution is calculated from the projections of the absorption at at least two different X-ray energies. From the relationship between two absorption values of the same point of the piece of luggage which are measured at different X-ray beam energies, at least approximate conclusions can be drawn concerning the atomic number of the substance situated there. (Generally, it is sufficient to be able to differentiate between a few atomic number ranges in order to be able to detect the presence of metals which have a high absorption, but which are not part of the substances to be detected.) By means of the atomic number and the absorption density distribution, a nuclear density distribution can then be calculated, and the first activity density distribution can be corrected such that the reconstruction of a more exact three-dimensional activity density distribution inside the piece of luggage becomes possible.
By means of such a precise activity density distribution, when the activation energies are known, conclusions can be drawn more reliably than previously concerning the presence of explosives and addictive substances. In particular, when there is a first suspicion, a piece of luggage can be subjected to a second examination at the suspicious points during which, primarily through excitation by means of an energy of between 15 and 18 MeV, the presence of oxygenous substances and of the concentrations of these substances that are typical for explosives, can be determined.
Furthermore, to improve the false-alarm rate, an X-ray tomograph in addition to the first X-ray unit, can be provided for the exact measuring of the atomic number and absorption density distribution. In this manner, only those pieces of luggage are examined which were classified as being suspicious on the basis of a first measurement. As a result, the average examining speed is reduced only slightly since the more extensive examination by means of the X-ray tomograph is carried out only at a few pieces of luggage which in the preliminary examination had been classified as suspicious.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.